For two-handed, reach weapons that you can use with Weapon Finesse, your options are basically spiked chain, and the drow scorpion chain (read: a slashing version of the spiked chain) from Races of Eberron.
Races of the Wild has several weapons that can be used with Finesse without being light, but none have reach. The lynxpaw is a double weapon (i.e. can be used as a two-handed weapon), and the elven courtblade just is a two-handed weapon. The elven thinblade is a one-handed weapon à la the rapier, i.e. it imposes restrictions on two-handing it.
Other rapier-like weapons include the ribbonweave from Races of Eberron and the quickblade rapier from Complete Adventurer.
The last such weapon I can find is the dragonsplit, from Monster Manual IV. These are one-handed but count as light for TWF and Finesse, and also have 19-20/×4 criticals. I’m not sure if they can be two-handed but since you’re not going for dual wielding I wouldn’t bother with them.
The spiked chain (or scorpion chain if you want slashing for some reason, or are a drow who can get easier proficiency) is far-and-away your best bet.
Alternatively, the feycraft template from Dungeon Master’s Guide II can make any one-handed weapon compatible with Weapon Finesse. It can also make light weapons use Dexterity instead of Strength without Weapon Finesse, which is often a bigger deal. I don’t know any one-handed weapons with reach, but the kusari-gama from Dungeon Master’s Guide is a light weapon with continuous reach à la spiked chain.
Yes.
Check the prerequisites. Power Attack needs a Strength 13, while Piranha Strike needs Weapon Finess, which is interesting only if your Dex is greater than you Str, so we can deduce that the latter is intended for Dexterity-oriented characters, the fluff text incidentally says one using Piranha Strike provokes "multiple, minor wounds".
For a more complete answer, Power Attack doesn't say anything about light weapons, but it does say that you can use this feat with a natural weapon, and natural weapons are considered light weapons.
Besides, if you couldn't use Power Attack with a light weapon, the description of the Piranha Strike feat wouldn't explicitly tell that
This feat cannot be used in conjunction with the Power Attack feat
Best Answer
Yes. You can use weapon finesse when attacking with your finger as an unarmed strke and when using it to deliver a touch attack.
Unarmed Strikes (your finger) counts as light weapons and qualify for weapon finesse.
From the d20pfsrd page about unarmed strike (emphasis mine)
However, This does not directly extend to making touch attacks. Touch attacks count as 'armed' unarmed attacks. Since unarmed attacks are considered light weapons, they qualify for weapon finesse.
http://paizo.com/prd/combat.html